Annual variation in event-scale precipitation δ²H at Barrow, AK, reflects vapor source region

Abstract. In this study, precipitation isotopic variations at Barrow, AK, USA, are linked to conditions at the moisture source region, along the transport path, and at the precipitation site. Seventy precipitation events between January 2009 and March 2013 were analyzed for δ²H and deuterium excess. For each precipitation event, vapor source regions were identified with the hybrid single-particle Lagrangian integrated trajectory (HYSPLIT) air parcel tracking program in back-cast mode. The results show that the vapor source region migrated annually, with the most distal (proximal) and southerly (northerly) vapor source regions occurring during the winter (summer). This may be related to equatorial expansion and poleward contraction of the polar circulation cell and the extent of Arctic sea ice cover. Annual cycles of vapor source region latitude and δ²H in precipitation were in phase; depleted (enriched) δ²H values were associated with winter (summer) and distal (proximal) vapor source regions. Precipitation δ²H responded to variation in vapor source region as reflected by significant correlations between δ²H with the following three parameters: (1) total cooling between lifted condensation level (LCL) and precipitating cloud at Barrow, ΔT_cool, (2) meteorological conditions at the evaporation site quantified by 2 m dew point, T_d, and (3) whether the vapor transport path crossed the Brooks and/or Alaskan ranges, expressed as a Boolean variable, mtn. These three variables explained 54 % of the variance (p<0. 001) in precipitation δ²H with a sensitivity of −3.51 ± 0.55 ‰ °C⁻¹ (p<0. 001) to ΔT_cool, 3.23 ± 0.83 ‰ °C⁻¹ (p<0. 001) to T_d, and −32.11 ± 11.04 ‰ (p = 0. 0049) depletion when mtn is true. The magnitude of each effect on isotopic composition also varied with vapor source region proximity. For storms with proximal vapor source regions (where ΔT_cool <7 °C), ΔT_cool explained 3 % of the variance in δ²H, T_d alone accounted for 43 %, while mtn explained 2 %. For storms with distal vapor sources (ΔT_cool > 7°C), ΔT_cool explained 22 %, T_d explained only 1 %, and mtn explained 18 %. The deuterium excess annual cycle lagged by 2–3 months during the δ²H cycle, so the direct correlation between the two variables is weak. Vapor source region relative humidity with respect to the sea surface temperature, h_ss, explained 34 % of variance in deuterium excess, (−0.395 ± 0.067 ‰ %⁻¹, p<0. 001). The patterns in our data suggest that on an annual scale, isotopic ratios of precipitation at Barrow may respond to changes in the southerly extent of the polar circulation cell, a relationship that may be applicable to interpretation of long-term climate change records like ice cores.